The present invention relates to optical fiber arrays and to apparatus and methods of grinding and polishing the face of each fiber in the array.
Advancements have been made in recent years in the art of design and manufacture of optical fiber arrays. Early optical couplers or connectors included between two to eight optical fibers secured in a single dimension or line of fiber ends by the body of the connector or coupler. The free face of each optical fiber could easily be ground and polished with use of standard equipment. Advancements led to two-dimensional arrays in which the fibers were secured in rows and columns, initially 4xc3x974 and 8xc3x978. Here to, the fiber faces were ground and polished by conventional apparatus, usually prior to insertion into the coupling array device.
The need in the telecommunications industry, however, is for arrays with much greater fiber positioning precision and far greater fiber density. Innovations such as disclosed in U.S. patent application Ser. No. 09/618,179 filed Jul. 18, 2000 contributed to achieving arrays with 19xc3x9719, 64xc3x9764, 128xc3x97128 fibers, and potentially higher fiber number arrays. Because of the physical requirements for arrays in the field, the packing density of these large numbers of fibers into an array has increased greatly thus creating technical problems in grinding and polishing tiny and closely packed optical fiber faces. These problems are made more complex by the desire to grind each fiber face at the same 8xc2x0 or other suitable angle to the normal of the respective fiber core axis. This angled face functions to prevent reflected optical energy (waves) from distorting the data content of the optical energy, which arises from a face ground perpendicular to the core axis.
An objective of the present invention is to provide an apparatus and method for polishing and/or grinding optical fiber ends in an array that solves the foregoing problems and provides other benefits and features some of which are described below.
Another objective is to grind and polish the face of each fiber in a high-density array at an angle (such as 8 degrees) from the plane perpendicular to the optical axis of the array, or other angle that may be desired. The purpose for preparing such an array includes reducing the back reflections from the fiber face when light is injected into the fiber from an optical element, such as a diode laser or any other device, in which back reflections can cause interference with the operation of that element.
One exemplary embodiment according to the principles of the present invention includes using an integral lapping tool (the lap) in which the surface of the lap has a saw tooth cross-section. The lap face can be composed of rows of tilted planes running parallel to each other on the lap surface. Each of these planes is tilted at the required angle to produce the desired angle on the fiber ends after polishing of the array end face. The spacing or pitch of the rows of these tilted planes match the pitch of the rows of the fibers in the array to be polished. The forward face of the array includes a layer of epoxy in which the fiber tips are embedded. The lap tool grinds the epoxy surface to form an angled surface in the epoxy as well as the fiber tips.
An alternate embodiment includes a series of thin plates with edges preground at the required angle, e.g., 8 degrees. These plates are stacked together giving a lap with the same type surface as above. That is, the face of this lap would have some rows of parallel planes tilted at 8 degrees and spaced a precise distance apart matching the pitch of the array.
In each of the above examples, the lap would be aligned with its tilted planes parallel to the rows of fibers in the fiber array. The lap would be moved across the face of the stationary array to be polished using a reciprocating lap motion. Alternatively, the face of the array could be drawn across the stationary lap using a reciprocating array motion.
As the lap is moved across the face of the array, or vice versa, a grinding and/or polishing compound, typically suspended in a carrying liquid, such as water, is introduced between the lap and the array face. Such grounding and polishing compounds can be typical of those commonly used in the optical grinding and polishing industry.
Another embodiment includes circular plates or disks each having its edge ground at the required angle from the vertical. The disks are stacked together to form a rotating plate assembly or tool bit. These circular lapping plates, preferably, with a diameter much greater than the individual fiber core diameter, can be rotated by a drive motor and, while rotating, simply drawn across the face of the fiber array that is to be ground and polished. The motion of the tool in this case would not be reciprocating but rotating in one angular direction, however, multiple translational passes across the multiple rows of fibers are required to achieve desired results. As before, a grinding or polishing compound, usually suspended in an appropriate medium such as water, kerosene, or other fluid would be introduced between rotating plates and the face of the array. Advantages of this system include (i) easier access of the grinding and polishing slurry to the space between lap surface and the surface of the array and fibers during grinding and polishing, and (ii) rotation of the tool tends to carry fresh slurry to the grinding/polishing surface and carry away the ground debris.
Yet a further variation of the lap includes plates with ground edges to give a truncated saw tooth cross-section rather that a pointed tooth cross-section. In the case of the integral lap, this would be accomplished by simply preparing the lap so that the rows have the desired truncated saw tooth profile. In the other case, the flat elements of the stacked plate lap or the circular stacked plates, each plate could have its edge modified by grinding its face with the desired profile or by making thinner plates with the full saw tooth edge and interleaving them in stacking with plates that have a vertical edge but have a smaller width or smaller diameter than the angled edge plates. In either case, the lapping tool can be formed by machining from a single piece of suitable material such as brass, steel, etc. in a lathe or NCM tooling unit. One advantage to this approach would be that no alignment constraints would be required as in stacking the layers of individual plates as mentioned above. Polishing efficiency and quality can be enhanced by vertically dithering the tool during rotation.
It will be understood that the number of plates could equal the number of rows (or columns) of the array or could be less than the number of rows (or columns). In the latter case, the rotating or reciprocating tool could be controlled in an indexed step-and-repeat method to grind one or more rows or columns at a time, then index to the next row or set of rows, and repeat until all rows or columns are ground or polished.
As mentioned above, the back reflections would cause interference with the device operation where the device is either launching light into the fiber in the array or accepting light from that fiber. Although similar results have been reported to be obtained by putting anti-reflection (AR) coatings on the ends of fibers, it has proven to be quite difficult to obtain a uniform coating across the full array of fibers. Although AR coating also reduces losses due to reflections, the effects due to interference and non-uniformity of the coating across the face of full array far outweigh the use of AR coatings. In contrast, the uniformity of effective back reflection across the end faces of a fiber array can be controlled very well by the grinding and polishing techniques according to the principles of the present invention.